This invention relates to telephone systems in general and more particularly to a line/interface circuit for use in a key telephone system.
When a given location, such as a business or office, has more than one line from the central office (CO) or from a private branch exchange (PBX) and a plurality of telephone stations, it is often desirable to equip the location with key sets to provide local switching functions and other desirable features. These features include:
(1) ability to pickup one or more of several central office lines;
(2) ability to hold one or more lines and use another line;
(3) ability to signal an associate;
(4) ability to have local intercommunication.
As can be readily ascertained, a wide variety of instruments are available for such uses ranging from a single line desk set to large ten, eighteen, twenty, thirty and more button sets or consoles. Audible and visual signals are required if there is more than one line and multiple access thereto. The audible signals are similar to central office ringing supply, while the visual devices comprise small lamps located under plastic key caps.
In order to employ such telephone key sets, a line circuit configuration is generally used to provide station control of line pickup and hold functions in such systems. Many suitable configurations exist in the prior art and have been available from various companies. As such, various United States patents show line circuits which may be employed in such systems and which provide various advantages.
For example, U.S. Pat. No. 4,013,844, entitled LINE CIRCUIT FOR A UNIVERSAL KEY/INTERNAL TELEPHONE SYSTEM, issued on Mar. 22, 1977 to B. Ronald Saxon and assigned to the Assignee herein, shows a balanced line circuit which operates from compatible power supplies found in conventional key service units. Other patents as U.S. Pat. Nos. 3,647,983, 3,941,943, and 4,075,434 show various line circuits which include different features to implement various operating modes.
A major problem in regard to conventional line circuits is the requirement that all such devices originate in telephone company provided equipment. Such equipment is sometimes designated as interface equipment and is installed and maintained by the telephone carrier. In the United States and elsewhere, FCC tariffs do not permit the direct connection of many prior art line circuits to telephone company facilities. Accordingly, the operating telephone company provides the interfaces to protect their facilities against hazardous voltages, imbalances, excessive signal levels and so on. The cost of these interfaces are added to the telephone bill and paid for by the consumer.
It is therefore desirable to provide a line circuit which will interface with a conventional telephone line, while providing complete isolation and hence, preventing the above described problems from occurring. Accordingly, the circuit to be described can be coupled directly to a telephone line and eliminate the need for the inclusion of an interface circuit.